This invention relates to an intraocular lens for implantation in the posterior chamber of a human eye and more particularly to such a lens for use with patients having both glaucoma and cataracts.
Intraocular lens implantation in either the anterior chamber or the posterior chamber of the human eye is a known technique for treating cataracts. Opacification of the human lens (the development of cataracts) is a common problem which becomes more acute with age. By the age of 70, approximately ninety percent of the population suffers substantial loss of vision due to lens changes. Although cataracts are most often associated with age, they are also associated with certain medical conditions such as diabetes, hypocalcaemia and uraemia.
To restore vision to a patient with cataracts, the lens of the eye is surgically removed. Once removed, another mechanism must be substituted to perform the focusing function of the lens. In the past, a thick spectacle lens or a contact lens has been used after cataract surgery. More recently intraocular lens implants have been used.
There are generally two types of intraocular implant lenses, anterior chamber lenses and posterior chamber lenses. Both types of intraocular lens use an optic with associated haptics. The haptics help to place and stablize the optic which performs the functions previously performed by the human lens.
Two different techniques can be used for lens extraction, intracapsular extraction or extracapsular extraction. Intracapsular extraction removes the entire lens structure. In contrast, extracapsular extraction removes the anterior capsule and the cortex material of the lens while leaving in place the posterior lens capsule. The posterior lens capsule is essentially a membrane. Anterior chamber lenses must be used when intracapsular extraction is performed. When extracapsular extraction is performed, although either a posterior chamber or an anterior chamber lens may be used, the lens of choice is the posterior chamber lens.
Generally, extracapsular extraction is preferred. This is especially so when the patient also suffers from glaucoma since the anterior chamber lens causes various complications in patients with glaucoma which are not caused by posterior chamber lenses. Additionally, extracapsular extraction has the advantage of providing a capsular support for the intraocular lens and this type of extraction diminishes complications resulting from vitreous loss, such as retinal detachment and cystoid macular edema.
Under normal circumstances, the ciliary body of the eye produces aqueous humor which supplies nutrients to the lens and the cornea and which maintains normal intraocular pressure. The aqueous humor circulates from the posterior chamber through the pupil and into the anterior chamber. It then drains through the trabecular meshwork into the Canal of Schlemm, and from there through the aqueous veins into the venous system.
Glaucoma is a symptom characterized by an increase in intraocular pressure associated with abnormalities in either the formation or the outflow of aqueous humor. Left untreated this increased pressure leads to optic nerve damage with corresponding visual field loss. It is estimated that 2% of the general population and 5% of the population over the age of 65 have glaucoma. Although medical treatment may be used effectively in many cases of glaucoma, frequently a trabeculectomy must be performed to create a fistula in the trabecular meshwork which permits free passage of aqueous humor from the anterior chamber into the subconjunctival space to alleviate pressure. In order for the surgery to be effective, the fistula must remain substantially unobstructed.
The occurrence of glaucoma in a patient with cataracts is not an unusual one in ophthalmology. Both are diseases that are age related and occur with high frequency in the elderly population. In one large series of patients undergoing cataract extraction, glaucoma was found to be present in 4%. It has been found that the incidence of both glaucoma and cataracts increases with age. Between the ages of 52 and 64, there is a 1.4% incidence of chronic open angle glaucoma and a 4.6% incidence of cataract or aphakia. Between the ages of 65 and 74, the incidence of glaucoma rises to 5.1% and that of cataract rises to 18.1%. In the next decade (75 to 85 years), the incidence of glaucoma is 7.2% and the incidence of cataract or aphakia is 46.1%. There is a progressive increase in the incidence of both these diseases during the process of aging. Several other factors further increase the association of cataracts and glaucoma. Previous surgery for glaucoma, whether either a peripheral iridectomy or glaucoma filtration surgery, will hasten the development of a cataract. The use of strong miotics such as phospholine iodide is also implicated in the development of cataracts. The use of miotics in treating glaucoma also induces an earlier onset of symptoms, because of a small pupil, in patients with early cataractous changes of their lens. This can make cataract surgery necessary at an earlier stage in many glaucoma patients. With approximately 1,200,000 patients undergoing cataract implant lens surgery during 1985, approximately 50,000 of these patients will have concomitant glaucomatous eye diseases.
While glaucoma had been considered earlier a relative contraindication to intraocular lens implantation, many reports have suggested that the use of extracapsular extraction and posterior chamber lenses do not interfere with post-operative control of glaucoma. Rather, studies have indicated that posterior chamber intraocular lens implantation had no effect on the incidence of post-operative intraocular pressure elevations. The incision in extracapsular extraction is smaller and hence disturbs less conjunctiva which may be used later in filtration surgery. Less trabecular meshwork is distorted by suture closure which may help minimize early post-operative pressure elevations. Implant lenses are beneficial for patients with glaucoma and visual field defects as the minimum magnification resulting from such lenses allow more visual information to be presented than would occur with magnification of a scotoma resulting from aphakic spectacles or contact lens correction.
Patients who have glaucoma are always at risk for developing an intolerance to medical therapy or laser therapy and may eventually require a filtration operation for control of their intraocular pressure. All these operations involve creating a fistula between the subconjunctival space and the anterior chamber. This is made in various ways by creating a hole at the limbus by either cutting out a portion of the limbal tissues with either a scalpel blade or creating this fistula by burning with a cautery through the subconjunctival space into the anterior chamber. Fluid then filters through this fistula and is either gradually absorbed by vessels within the conjunctiva or gradually filters through the conjunctival tissues to be extruded externally with the tears. Planning and undertaking cataract surgery in an eye with glaucoma is extremely important because the management of glaucoma in the aphakic eye is difficult and frequently unsuccessful. One of the major anti-glaucomatous medications, epinephrine, will cause in 20% of aphakic patients cystoid macular edema. While this maculopathy is usually reversible in the early stages, this must be considered a relative contraindication to the use of this drug in patients with glaucoma who are aphakic. The limitation of medical options only makes this problem more difficult because the surgical treatment of aphakic glaucoma has an extremely low success rate. Several studies on the success rate of standard filtration surgery range from 25 to 60%. One study with the success rate of 75% combined trabeculectomy surgery with a total vitrectomy. All these studies have evaluated patients whose aphakia was intracapsular in origin. Patients treated now and in the future will more likely have an extracapsular aphakia, and more probably an extracapsular aphakia with a posterior chamber lens. The cause of the poor results of filtration surgery in intracapsular aphakic glaucoma are not clear. Three possible reasons are proposed: 1. scarring of the conjunctival tissue resulting in inadequate bleb formation for filtration of fluid through the conjunctiva; 2. mechanical plugging of the filtering fistula by vitreous and/or ciliary processes; and 3. technical difficulties because of the complexity of the surgery.
The use of extracapsular surgery has brought hope that some of these problems can be alleviated. Separation of the vitreous from the aqueous by the posterior capsule may inhibit some induced change in the aqueous humor that makes it more fibrogenic in nature. This may reduce the incidence of scarring in the subconjunctival space and lead to better filtering of fluid and better bleb formation in the conjunctiva. The presence of a posterior capsule that is intact has been theoretically proposed as an effective barrier to the movement of vitreous which can cause an external plugging of the sclerostomy or the fistula site.
The posterior capsule after extracapsular surgery is diaphenous and extremely mobile. The posterior capsule and the vitreous which sits beneath it is readily moveable and will frequently create adhesions to the iris without the presence of an implant lens. In fact, patients with high myopia are recommeded by many to have an implant lens not for optical correction but to prevent the mobility of the posterior capsule. This mobility and subsequent anterior-posterior movement of the vitreous removes many of the benefits of extracapsular surgery. While the 6 to 7 mm. optic of a posterior chamber lens prevents this mobility centrally, it does nothing for this mobility in the far periphery of the posterior capsule. This is the area of concern in patients with glaucoma for this is where the fistula site is created during the performance of a glaucoma filtration operation. Here in the periphery, the mobility of the posterior capsule can readily plug a filtration site. In fact, if all the cortex is not removed during an extracapsular operation it too can plug and occlude a fistula site during concurrent or subsequent glaucoma filtration surgery. This plugging of the fistula site can be an important factor in the disappointing results in aphakic filtration surgery. The biggest difference between filtration surgery in phakic and aphakic eyes is the lack of the normal lens-iris diaphragm in the periphery that exists in phakic eyes undergoing filtration surgery. The success rate of filtration surgery in phakic eyes is estimated from 75 to 95%.
It is evident that extracapsular extraction with a posterior chamber lens is desirable in patients with glaucoma who must undergo cataract surgery. Certainly, it has been shown that the results of filtration surgery in patients with aphakia of the intracapsular type is extremely disappointing. Scarring at the conjunctival site is a problem that is largely being solved with the use of antimetabolities such as 5-fluorouracil injected subconjunctivally. Plugging of the filtration sites is a problem that has not been solved by extracapsular surgery. Structures in the eye that can block this filtration site include the ciliary processes (this occurred also with intracapsular aphakia), retained cortical material, and the posterior capsule itself which remains mobile in the periphery.
Accordingly, it is a purpose of this invention to provide a posterior chamber intraocular lens which can be effectively used with glaucoma patients.
It is another purpose of this invention to provide such a lens which prevents the posterior capsule and ciliary process from occluding a fistula created in the trabecular meshwork.
Still another purpose of this invention is to provide such a lens, which is easily usable by surgeons and which does not create any additional complications for patients.